1. Field of the Invention
The present invention relates to an information processing terminal and a received voltage controlling method.
2. Description of the Related Art
Recent years have seen widespread acceptance of information processing terminals such as noncontact IC (integrated circuit) cards (called the IC card hereunder) and RFID (radio frequency identification) tags capable of noncontact communication with reader/writers.
The IC card and RFID tag have no power sources of their own; they are driven by magnetic-field energy obtained from the reader/writer. More specifically, the reader/writer has a current flowing through its transmitting coil acting as a transmit-receive antenna so as to generate a magnetic field. The IC card and RFID tag are each included with a receiving coil serving as a transmit-receive antenna. When the magnetic field of the reader/writer is transited by the IC card or RFID tag with its receiving coil, the coil induces a voltage (called the induced voltage) in reaction to the magnetic field. The voltage thus generated serves to power the IC card and RFID tag.
The IC card and RFID tag communicate with the reader/writer emitting a magnetic field of a particular frequency such as 13.56 MHz. The ID card and RFID tag are each included with a resonance circuit set to resonate with that specific frequency. In operation, the IC card or RFID tag using its receiving coil acquires magnetic field energy from the transmitting coil of the reader/writer and receives a voltage (called the received voltage) by causing the induced voltage to resonate with the predetermined frequency.
In general, the shorter the distance between the reader/writer and the IC card or RFID tag, the higher the intensity of the magnetic field received by the IC card or RFID tag from the reader/writer. The level of the received voltage is also higher the shorter that distance.
Conversely, the longer the distance between the reader/writer and the IC card or RFID tag, the lower the intensity of the magnetic field received by the IC card or RFID tag from the reader/writer. The level of the received voltage is also lower the longer that distance.
If there exists an obstruction (e.g., another IC card) between the reader/writer and the IC card or RFID tag, the resonance frequency decreases and so does the received voltage.
Against the circumstance outlined above, efforts have been made to develop an information processing terminal capable of operating in a stable manner regardless of the varying level of the received voltage. As part of such efforts, Japanese Patent Laid-open No. 2001-222696 discloses a technique for stabilizing the performance of information processing terminals. Japanese Patent Laid-open No. 2004-46292 discloses a technique used by the reader/writer for controlling the power to be supplied to information processing terminals.